The present invention relates to an optical sensor for viewing earth from an orbit thereabout and determining positional information of the sensor relative to earth by observing the limb of the earth. The invention further relates to a method for determining the rate and direction of rotation of an initially tumbling satellite about all three axes of the satellite, for eliminating the tumbling motion and stabilizing the satellite in a desired orientation, and for sensing the orientation of the stabilized satellite about two of the three axes, all using a single optical sensor.
Optical sensors for satellites have been developed for viewing earth in order to derive position information of the satellite relative to earth. In such schemes, it is known to direct light from a field of view of the sensor onto a focal plane array, such as a charge coupled device (CCD), comprising a grid of pixels. The field of view and the optics of the sensor are typically designed such that at least part, and more typically all, of the circumference of the earth""s limb (i.e., the transition region between the earth and space) can be imaged onto the focal plane when the sensor is pointed in a suitable direction relative to the earth. The relative location of the image of the earth limb on the focal plane is determined by finding the pixels at which a large gradient in intensity of the incident light energy, indicating a transition between earth and space, is located. Using an appropriate algorithm, it is possible to determine the rotational orientation of the sensor, and hence of the satellite, about two orthogonal axes based on the locations of the transition pixels of the focal plane array. See, for example, U.S. Pat. No. 6,026,337.
A number of patents for various types of optical sensors have been acquired by the assignee of the present application, including U.S. Pat. Nos. 5,502,309, 5,534,697, 5,627,675, and 5,841,589, the entire disclosures of which are hereby incorporated herein by reference. The sensors described in all of the aforementioned patents have a single field of view for looking at the limb of the earth. On satellites using a limb-looking optical sensor as described above, the optical sensor is generally used for deriving position information about two axes after the satellite has been stabilized following tip-off from the launch vehicle. An entirely different system is used for bringing the initially tumbling satellite into a controlled condition and stabilizing the satellite in that condition. Typically, inertial measurement units (IMUs), i.e., gyroscopes, are used for detecting the rotation rate of the tumbling satellite about all three axes, and this rate information is used by the satellite""s control system to stop the tumbling motion. Once the satellite is no longer tumbling, it is then manipulated to place it in the desired orientation. Still other sensors are typically used to aid in this process, since it is possible for the satellite to be brought to a stabilized condition in an orientation in which the limb-looking sensor is looking away from earth so that the earth is not in its field of view. For instance, a sun sensor and/or sensors for viewing stars or other celestial bodies may be used to aid in maneuvering the satellite toward the desired orientation, at least until the earth comes into the field of view of the limb-looking sensor. It is apparent that this approach requires a considerable number of sensing devices.
The present invention provides an optical sensor and a method for determining the rate and direction of rotation of an initially tumbling satellite about all three axes of the satellite, for eliminating the tumbling motion and stabilizing the satellite in a desired orientation, and for sensing the orientation of the stabilized satellite about two of the three axes, all using a single optical sensor. The invention thus allows the usually required IMUs and sun or star sensors to be eliminated, thereby providing substantial savings in weight, cost, and complexity.
To these ends, an optical sensor in accordance with a preferred embodiment of the invention comprises at least one focal plane array comprising a plurality of pixels arranged in a grid, and both panoramic optics and limb-looking optics each of which maps its field of view onto a separate region of the at least one focal plane array. The panoramic optics capture radiant energy from an annular panoramic field of view spanning 360xc2x0 in azimuth angle about an optical axis of the sensor and covering a range of elevation angles including 90xc2x0 in elevation angle relative to the optical axis. The panoramic optics re-direct and focus the radiant energy from the panoramic field of view onto a first annular region of the at least one focal plane. The limb-looking optics capture radiant energy from an annular field of view spanning 360xc2x0 in azimuth angle about the optical axis and covering a range of elevation angles non-perpendicular to the optical axis such that at least a major circumferential portion of a limb of the earth is within the field of view of the limb-looking optics when the optical axis of the sensor points toward a centroid of the earth. The limb-looking optics re-direct and focus the radiant energy onto a second annular region of the at least one focal plane.
A particularly simple sensor construction is made possible by directing the radiant energy from both fields of view onto the same focal plane array. Preferably, the panoramic field of view that looks generally perpendicular to the optical axis is imaged onto an inner ring-shaped region of the focal plane array, and the field of view that looks non-perpendicular to the optical axis is imaged onto an outer ring-shaped region radially outward of the inner ring-shaped region. Alternatively, where redundancy is desired for enhanced reliability, either or both of the fields of view can be optically split and imaged onto more than one focal plane array. Redundant electronics can be provided for the various focal plane arrays if desired.
Various optical arrangements for the sensor can be used. In one embodiment, the panoramic optics include a convex mirror of generally annular form that re-directs the radiant energy from the panoramic field of view along a direction generally parallel to the optical axis and away from the focal plane array, and a concave mirror that receives the radiant energy from the convex mirror and re-directs the radiant energy back generally toward the focal plane array. A curved meniscus lens receives the radiant energy from the concave mirror at a central portion of the meniscus lens, and a final lens receives the radiant energy from the central portion of the meniscus lens and focuses the radiant energy on the first annular region of the focal plane array. The limb-looking optics re-direct the radiant energy from the field of view of the limb-looking optics onto an outer portion of the curved meniscus lens lying radially outward of the central portion thereof, and the outer portion of the meniscus lens serves as a final optic for focusing the radiant energy onto the second annular region of the focal plane array. Preferably, the limb-looking optics comprise a plurality of lenses.
In another embodiment of the sensor, the convex mirror re-directs radiant energy from the panoramic field of view generally inwardly and toward the focal plane array, and the panoramic optics include a first lens having a central portion that receives radiant energy from the convex mirror, and a second lens that receives radiant energy from the central portion of the first lens and focuses the radiant energy onto the first annular region of the focal plane array. This embodiment thus eliminates the concave mirror of the previously described embodiment.
In still another embodiment of the sensor, the panoramic optics include a convex mirror that re-directs radiant energy from the panoramic field of view generally inwardly and toward the focal plane array, a pair of lenses in series that receive radiant energy from the convex mirror, and a curved meniscus lens having a central portion that receives radiant energy from the pair of lenses and focuses the radiant energy onto the first annular region of the focal plane array. The limb-looking optics comprise lenses for directing radiant energy from the field of view of the limb-looking optics onto an outer portion of the curved meniscus lens, the outer portion of the curved meniscus lens serving as a final lens for focusing the radiant energy onto the second annular region of the focal plane array. The pair of lenses preferably comprise two meniscus lenses with a concave surface of one of the meniscus lenses facing a concave surface of the other meniscus lens.
The invention also provides a method for determining the direction and rate of rotation of a satellite about first, second, and third mutually orthogonal body axes fixed relative to the satellite and for determining orientation of the satellite relative to earth. In accordance with this aspect of the invention, a dual field-of-view optical sensor is mounted on the satellite with an optical axis of the sensor in a predetermined orientation with respect to the body axes of the satellite. Radiant energy is directed onto a first annular region of a focal plane array from a first field of view of the sensor spanning 360xc2x0 in azimuth angle about the optical axis and a range of elevation angles including 90xc2x0 in elevation angle relative to the optical axis. Radiant energy is directed onto a second annular region of the same or a different focal plane array from a second field of view of the sensor spanning 360xc2x0 in azimuth angle about the optical axis and a range of elevation angles non-perpendicular to the optical axis. Direction and rate of rotation of the satellite about the body axes are determined based on changes in the relative location of the earth limb subtense on the first annular region of the focal plane array over time. The sensor electronics periodically take readings from the focal plane array(s), and the location of the earth limb on the focal plane array(s) is compared at successive times to derive the direction and rate information. Orientation angles between the body axes of the satellite and a nadir vector to the earth are determined based on the relative location of the earth limb subtense on the second annular region of the focal plane array(s). In the general case, the earth limb location on the second annular region of the focal plane array can be used for deriving rotational orientation about two axes. The rotational orientation of the satellite about the third body axis can be determined by directional radio reception (e.g., from sister satellites in a constellation), by directional magnetic fields, or by directional inertial rotation.